Automated analyzers commonly use pipettes to perform biological tests on a series of biological samples (e.g., urine specimens). An automated analyzer uses the pipette to cyclicly perform a sequence of steps. In each sampling cycle, the pipette first aspirates a biological sample from a sample tube and dispenses a portion of the sample onto a strip of multiple reagent pads. Next, if desired, the pipette dispenses the remaining portion of the sample into a specific gravity well in order to measure the specific gravity of the sample. Finally, the pipette is self-cleansed of residual sample by expelling cleaning solution, coupled to the proximal end of the pipette, from the distal tip thereof into a discharge basin. To clean the exterior of the pipette, the discharge basin may be constructed in the shape of a well sized to receive the pipette tip. The cleaning solution expelled from the pipette tip is forced by the well to surround the exterior of the tip, thereby cleaning it. At this point in time, the sampling cycle has been completed and the automated analyzer is ready to execute another cycle on another biological sample.
In order to detect when the pipette enters the sample in the sample tube, the pipette may be constructed with fluid sensing capabilities. One such fluid sensing pipette is described in U.S. Pat. No. 3,754,444 to Ure et al. This pipette includes an inner conductive tube forming a first electrode and an outer conductive tube forming a second electrode. The outer conductive tube is disposed concentric about the inner conductive tube and is insulated from the inner conductive tube by a plastic tubular sleeve. The inner and outer conductive tubes are coupled, by means of spaced wires, to level detection circuitry. When the inner and outer conductive tubes enter a sample within a sample tube, the level detection circuitry senses a change of the impedance between air and the sample and signals a motor to stop the descent of the pipette.
To maximize the efficiency at which biological tests are performed in an automated system, it is preferable to minimize the time period in each sampling cycle. To aid in this minimization of cycle time, the time period for cleansing the pipette tip should accordingly be minimized. A drawback of the foregoing fluid sensing pipette is that it is difficult to cleanse both the interior and exterior of the pipette in a relatively short period of time and with a minimal volume of cleaning solution. In particular, the outer conductive tube must be designed with a large enough diameter to accommodate the insulative plastic sleeve disposed between the inner and outer conductive tubes. The presence of the plastic sleeve significantly increases the required diametric size of the outer conductive tube which, in turn, increases the outer surface area of the outer conductive tube. Due to the increased outer surface area, a relatively large amount of cleaning solution would need to be expelled from the pipette tip in order to cleanse the exterior of the pipette tip. This would be a time-consuming operation.
Another drawback of the foregoing pipette is that the pipette may cause cross-contamination between different pipetted fluids because aqueous drops or constituents (e.g., proteins) of one pipetted fluid may carryover to the operation of the pipette on a different fluid. This cross-contamination may occur even though the pipette is cleansed with cleaning solution after it aspirates and dispenses each pipetted fluid. The reason for this is that aqueous drops of pipetted fluids strongly adhere to the inner surface of the inner conductive tube. Moreover, constituents such as proteins preferentially bind to the inner surface of the inner conductive tube.
Thus, a need exists for a fluid sensing pipette which overcomes the abovenoted drawbacks associated with the foregoing pipette.